.Net why does Threading.Task.Task still block my UI?

I'm currently writing an application that will control positioning of a measurement device. Because of the hardware involved I need to poll for the current position value constantly while running the electric motor. I'm trying to build the class responsible for this so that it does the polling on a background thread and will raise an event when the desired position is reached. The idea being that the polling won't block the rest of the application or the GUI. I wanted to use the new Threading.Task.Task class to handle all the background thread plumbing for me.

I haven't got the hardware yet, but have build a test stub to simulate this behavior. But when I run the application like this the GUI still blocks. See a simplified example of the code below (not complete and not using separate class for device control). The code has a sequence of measurement steps, the application has to position and then measure for each step.

public partial class MeasurementForm: Form
{
    private MeasurementStepsGenerator msg = new MeasurementsStepGenerator();
    private IEnumerator<MeasurementStep> steps;

    // actually through events from device control class
    private void MeasurementStarted()
    {
        // update GUI
    }

    // actually through events from device control class
    private void MeasurementFinished()
    {
        // store measurement data
        // update GUI
        BeginNextMeasurementStep();
    }

    private void MeasurementForm_Shown(object sender, EventArgs e)
    {
        steps = msg.GenerateSteps().GetEnumerator();
        BeginNextMeasurementStep();
    }        
    ...
    ...

    private void BeginNextMeasurementStep()
    {
        steps.MoveNext();
        if (steps.Current != null)  
        { 
            MeasurementStarted();
            MeasureAtPosition(steps.Current.Position); 
        }
        else    
        { 
            // finished, update GUI
        }
    }

    // stub method for device cont